Tractor beam built from rings of laser light

AT LAST we have a tractor beam that can move particles around, and can do the job in space as well as on Earth. Built with twin lasers, the device might not compare to anything out of Star Trek, but it has grabbed NASA's attention as it could one day prove useful for exploring other worlds.

Lasers are a standard way to build optical "tweezers" that can pick up a microscopic object and put it down a short distance away. A more recent device is able to pull on particles, but it relies on sensitive temperature variations in the beam, making it hard to deploy in space.

Now David Ruffner and David Grier of New York University have combined two Bessel beams - lasers that put out light in concentric rings - using a lens that focuses them so that they overlap. This creates a pattern of alternating bright and dark regions along the length of the beam. Fine-tuning the beam causes photons in the bright regions, initially flowing past a chosen particle in the beam, to scatter backwards. When these photons hit the particle, they knock it to the next bright region. The particle is thus constantly pushed closer to the beam's source.

Ruffner and Grier used this set-up to move microscopic silica spheres suspended in water over distances of around 30 micrometres (Physical Review Letters, doi.org/jkb).

This tractor beam arrangement acts more effectively on a wider range of microscopic objects than other set-ups, says David McGloin of the University of Dundee, UK, who was not part of the team. It might therefore be useful for collecting dust or atmospheric particles from other worlds for analysis.

"NASA contacted us," says Ruffner. "They were wondering, can we put this on a space probe and get dust from a comet?"

It is possible, he says, but not any time soon. "This is still very much in its infancy."

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